kinematics vs kinetics

Watch a film of someone running and you can describe everything you see: how fast their knee bends, the angle of the ankle, the path of the foot. But the film alone never shows you the forces — how hard the foot slammed into the ground, how much the muscles had to pull. Biomechanics splits movement into exactly these two questions: what the motion looks like, and what forces caused it.

Kinematics is the description of movement without reference to the forces causing it — positions, angles, speeds, and accelerations. It answers 'what happened': the knee flexed to 60 degrees, the foot swung through at this velocity. Kinetics is the study of the forces and torques that produce or result from that movement — the push of the ground, the pull of muscles, the load on a joint. It answers 'what made it happen'. A simple way to keep them apart: kinematics is what a slow-motion video can show you; kinetics needs a force plate or a calculation, because forces are invisible. The same visible motion can be produced by very different internal forces, which is why two people can walk with an identical-looking limp for completely different reasons.

Both are needed, and confusing them misleads treatment. A child and an adult may show the same kinematic gait pattern, yet the forces (kinetics) loading a small joint differ enormously. After surgery a knee may look like it bends normally (good kinematics) while the muscle forces controlling it (kinetics) are still far too weak for safe stair-climbing. Kinematics tells you the shape of the problem and is easy to observe; kinetics tells you the cause and the load, and often explains why a movement that looks fine still is not safe or efficient.